Yes you don’t have to use the same pension company for any AVCs. You can establish a separate Stand Alone PRSA AVC Plan. But that may involve higher costs (including set-up costs). I don’t think the diversification benefit would justify the additional costs.
By contributing extra contributions to your current scheme, it may be the case that you will only pay the underlying fund management charge (no set-up costs). You should be able to invest the additional contributions in a different fund(s) if you want to change the investment profile. The typical charges in a Group DC tend to be more competitive than establishing a separate AVC plan
So I would start with your existing scheme, find out what the costs will be if you simply up your contributions. In such a case I don’t think there should be any up-front costs, just the ongoing fund management charge.