Here's how I searched how to do this:
The way to add a table in Docx is to use the <w:tbl> tag. So I searched for this in the github repository, and found it in this file (called Writers/Docx.hs, so it's not a big surprise)
blockToOpenXML opts (Table caption aligns widths headers rows) = do
  let captionStr = stringify caption
  caption' <- if null caption
                 then return []
                 else withParaProp (pStyle "TableCaption")
                      $ blockToOpenXML opts (Para caption)
  let alignmentFor al = mknode "w:jc" [("w:val",alignmentToString al)] ()
  let cellToOpenXML (al, cell) = withParaProp (alignmentFor al)
                                    $ blocksToOpenXML opts cell
  headers' <- mapM cellToOpenXML $ zip aligns headers
  rows' <- mapM (\cells -> mapM cellToOpenXML $ zip aligns cells)
           $ rows
  let borderProps = mknode "w:tcPr" []
                    [ mknode "w:tcBorders" []
                      $ mknode "w:bottom" [("w:val","single")] ()
                    , mknode "w:vAlign" [("w:val","bottom")] () ]
  let mkcell border contents = mknode "w:tc" []
                            $ [ borderProps | border ] ++
                            if null contents
                               then [mknode "w:p" [] ()]
                               else contents
  let mkrow border cells = mknode "w:tr" [] $ map (mkcell border) cells
  let textwidth = 7920  -- 5.5 in in twips, 1/20 pt
  let mkgridcol w = mknode "w:gridCol"
                       [("w:w", show $ (floor (textwidth * w) :: Integer))] ()
  return $
    [ mknode "w:tbl" []
      ( mknode "w:tblPr" []
        ( [ mknode "w:tblStyle" [("w:val","TableNormal")] () ] ++
          [ mknode "w:tblCaption" [("w:val", captionStr)] ()
          | not (null caption) ] )
      : mknode "w:tblGrid" []
        (if all (==0) widths
            then []
            else map mkgridcol widths)
      : [ mkrow True headers' | not (all null headers) ] ++
      map (mkrow False) rows'
      )
    ] ++ caption'
I'm not familiar at all with Haskell, but I can see that the border-style is hardcoded, since there is no variable in it:
let borderProps = mknode "w:tcPr" []
                    [ mknode "w:tcBorders" []
                      $ mknode "w:bottom" [("w:val","single")] ()
                    , mknode "w:vAlign" [("w:val","bottom")] () ]
What does that mean ?
That means that you can't change the style of the docx tables with the current version of PanDoc. Howewer, there's a way to get your own style.
How to get your own style ?
- Create a Docx Document with the style you want on your table (by creating that table)
 
- Change the extension of that file and unzip it
 
- Open 
word/document.xml and search for the <w:tbl>  
- Try to find out how your style translates in XML and change the borderProps according to what you see.
 
Here's a test with a border-style I created:

And here is the corresponding XML: 
<w:tblBorders>
  <w:top w:val="dotted" w:sz="18" w:space="0" w:color="C0504D" w:themeColor="accent2"/>
  <w:left w:val="dotted" w:sz="18" w:space="0" w:color="C0504D" w:themeColor="accent2"/>
  <w:bottom w:val="dotted" w:sz="18" w:space="0" w:color="C0504D" w:themeColor="accent2"/>
  <w:right w:val="dotted" w:sz="18" w:space="0" w:color="C0504D" w:themeColor="accent2"/>
  <w:insideH w:val="dotted" w:sz="18" w:space="0" w:color="C0504D" w:themeColor="accent2"/>
  <w:insideV w:val="dotted" w:sz="18" w:space="0" w:color="C0504D" w:themeColor="accent2"/>
</w:tblBorders>
What about odt ?
I didn't have a look at it yet, ask if you don't find by yourself using a similar method.
Hope this helps and don't hesitate to ask something more