Addon domain ftp

I tried to add an addon domain in Hostgator but the subdomain/ftp username field wont let me and I read that there is a 25 character limit for that

So the problem again for me is that my domain name is a lot greater than 25 characters

I finally got the enthusiasm to get a second site uploaded but now its this too many letters domain to even add an addon domain.

Anyone have a solution apart from a new shorter domain as really I need te exact domain I have?

Thanks