I guess I'll start posting here, lol. If yall wondering suddenly, just spreading my internet prescence.
Make sure to check out my links to check out previous works cause I'm too lazy to do years worth of upload backlog unless weasyl has an ez FA import!!!!
I'm pretty sure there were some tools that would help you post your existing submissions on other sites to Weasyl, but they're at least a few years old and I don't know if they still work. This page mentions one of them (Ottermatic) but the link doesn't go anywhere now.
I know the Weasyl admins will respond to email at support (at) weasyl.dev within a day or two, so it might be worth asking them if they know of any current import tools. (That address is also linked from the bottom of each page, under "Support".)
I don't work for Weasyl or volunteer for them.
Link
NecroDrone
Most people use Postybird to cross post to other sites, but as far as some sort of backlog upload, I have no idea. Welcome! :3