You can do this using field codes.
Press Alt+F9 to toggle displaying the field code and the calculated value. You'll notice that when you insert a page number, it's actually { PAGE }.
Note that the curly braces { } can't be typed, they must be inserted using Ctrl+F9.
What we need to do is use a formula. You want to offset the number by 5, so you want something like:
{ = { PAGE } + 5 \*ROMAN }
To insert this:
Press Ctrl+F9 twice to get { { } }
Type PAGE inside the inner braces
Type the = between the first two (opening) braces
Type + 5 \*ROMAN between the last two (closing) braces
Press Alt+F9 to view the result
Explanation:
The = denotes a formula field. It's similar to an Excel formula, though a little more basic.
The { PAGE } inserts the current page number into the formula - the inner field is calculated first. If you were on the first page, the formula field would see something like { = 1 + 5 \*ROMAN }. If PAGE were not denoted as a field code by enclosing it in braces, you insert the text PAGE into the formula field which interprets that as a bookmark.
The addition should be obvious.
The \* specifies the displayed format. ROMAN means upper case Roman numerals, roman means lower case Roman numerals. It defaults to Arabic numerals if you don't specify a format.