If I'm gonna use HCG while still using steroids, will it improve my sperms fertility?
In theory yeah, but it would increase you sperms count by so little that it probably won't matter and you won't be able to get her pregnant.
If you're using steroids, then your sperm count is lowered, production is suppressed and HCG won't help. That's why people stop using steroids when they are ready to get their woman pregnant.
It is possible to get a girl pregnant while on steroids but I don't think that HCG would help, it would increase sperm count but not enough. For some people it will be impossible but for some very possible! I always use some kind of contraceptive even when I'm on steroids, because it's still not 100% safe! And I don't want to be a daddy just yet.