I'm not sure it would be a good idea to have it as a paid service. Since this will be one of the first things users will see when interacting with the platform, I don't think it would be appropriate to ask them right off the bat. The registration process is one of the most critical parts of an application, since all the users have to go through, and it's important to remove as much friction as possible there.
Offering the option after the sign up process would clash with the idea of this representing a unique, immutable identity. So I would just decide if it's possible to choose the emojis or not, but I wouldn't complicate it further (I think it'll already be complicated for most users to understand what that field means).