Если в GET, то кажись для RewriteRule есть ключик *|QSA|*
'*|qsappend|QSA|*' (*q*uery *s*tring *a*ppend)
This flag forces the rewriting engine to append a query string part in
the substitution string to the existing one instead of replacing it. Use
this when you want to add more data to the query string via a rewrite rule.