Hi Piotr,
sorry for the late response, I don't visit the AAA forum very frequently - as a general recommendation if you have VPN+AAA related questions it might be better to post them in the forum.
Anyway, do you still need help with this? I'm not sure if I understand the setup entirely. When you say "ASA is a authenticator for employees by L2L IPsec using IAS Radius" do you mean Client VPN (aks remote access vpn) instead of L2L?
If so, then enabling "password-management" in the tunnel-group will switch from PAP to MSCHAPv2.
If on the other hand you mean that you are using proxy authentication (aaa authentication include ...) then I don't think this is possible, although in that case you may want to double check in the forum.
hth
Herbert