Yes, you can roll most IRA, SEP IRA, SIMPLE IRA (after 2 years), ROTH IRA (only into a Roth Solo 401k, 457(b), 403(b), Profit Sharing and other Solo 401k’s into your Solo 401k.
An IRA can be rolled into a Solo 401k if the funds in it were rolled over from a previous 401k, 403b or 457. If this is the case your IRA is considered a conduit IRA and can be rolled over into a Solo 401k. If funds in your IRA came from you directly you are not permitted to roll the IRA into a Solo 401k.
In some instances you may be able to roll funds from a 401k at a current employer through what’s called an “In-Service Distribution,” although most large employers are highly restrictive and make this nearly impossible.