You are right, it doesn't accept these characters.
We are going to review this for the upcoming version and will add the possibility to store a SIP URI.

However please note that usually you just need to store the SIP username and the softphone will use the configured SIP server as the domain part. Can't this solve your problem? How exactly do you use the softphone? (The normal usage would be to just configure one or more SIP accounts and place calls by using only username or extension number. Or do you wish to call directly to many different SIP servers?)