Taxable Event. There's no limit on the amount you can roll to a Roth from another retirement account. However, a Roth rollover is a taxable event. Traditional IRAs, 401k plans, 403b plans and 457 plans are typically composed of pretax dollars.Roth IRA contributions come from after-tax dollars.So when you effect the rollover, you have to pay taxes on the money that was originally in a pretax account.ou can move money from nearly any retirement account into a Roth. The list of eligible account types includes the 401k, 403b, 457 plan, traditional IRA, SIMPLE IRA and SEP-IRA. You can even roll money from your employee profit-sharing, money purchase and defined benefit plans.