It's not do-able in any way, shape or form.
They are 2pc, a centre, which is bolted to a complete rim.
The rim is set at 7" wide...no options (well, you could fit Mk4 RXII rims, at 6.5" wide) so no, you can't make them wider.
They aren't a split rim, in the true context...they are a multi piece, or two piece.