If you are talking just the vent cover (And not adding a new fan etc) yes, you can put a larger cover over it. I just did this myself on my Leocraft,
The bathroom vent is only 9 inches and it isn't coming out of there without a lot of effort and major rebuilding.
I installed the large Maxx Air cover over it using the standard methods, however, there is not relly anything but
fiberglass to bolt it down to. So, in addition to the butyl and Dicor and the screws into glass, I used
Eternabond RoofSeal 2" x 4' Mini-Roll / White
not only as a sealant, but as a fastener to hold the vent down. I have used this stuff before, and believe me, that cover is going nowhere.
The bad part about doing this is.. it is staying put until major effort to get the sealant off is expended. I'll take the super sealant factor any day, however.