The premium plugin offers a custom CSS section (under question 6) where you can define your own font type.
Please insert there:
.sfsiaftrpstwpr .sfsi_plus_Sicons span:first-child, .sfsibeforpstwpr .sfsi_plus_Sicons span:first-child{
font-family: <font that user wants to apply>;
}
For example:
.sfsiaftrpstwpr .sfsi_plus_Sicons span:first-child, .sfsibeforpstwpr .sfsi_plus_Sicons span:first-child{
font-family: Raleway;
}