[Exim4U] add alias in a group

Hi everyone, exim4u works very well. I have exim4u 3.1.0 on ubuntu 16.04. I have a group with many members, i want add a member with different address from my domain but group accept only local mail address. I create a local address that forward to different domain and add this to the group but with forward the destination mail system doesn't accept any messages cause spf mismatch. Then i change mail address to alias and in this way the destination receive mails but i can't add alias in a group. Have an idea? Can i add a row in group_contents table? Luca