as title really, can i convert quick release forks to bolt through?

cheers

No.

Well not 15 or 20mm bolt through. You can get a 9mm sort of bolt through for some wheels

No

if there's a version of the same fork with a bolt through lower yes just swap and refill with the correct amount of oil/air

not in a way that would be much cheaper than buying a new fork … or at leeast a set of lowers

foxs yes – get the lowers changed for 15mm and use suitable adaptors for suitable hubs.
otherwise, with the same lowers, you can use 9mm conversion kits to adapt hope pro11 to 9mm such as the dt swiss rws 9mm, or do teh same with the dt swiss 240 hubs, or use a hub like the on-one 456 hub or the spesh stout.

yes. source a solid cr-mo axle with axle nuts and convert the hub itself.

Or just get some hex skewers which you can tighten up a lot more than qrs and have a similar effect. Lighter too, and cost about a tenner.

youll need to get a 9mm qr or thru axle like dt swiss(thread a week or so ago about cheap diy ones) or a threaded 9mm bolt & nuts
i got a chumba one…..

and if you have a compatible hub; dt swiss, spesh stout, pro II, some bontrager, some halo ones, get the apropriate adaptor and it fits in your qr dropout like so

and it will noticeably stiffen up 32mm foxes if you ride hard