Themis ,
1) Is the jsmpp the appropriate library for the implementation of a
smpp server?
Haven't tried but it is solid enough you can use it . It has limited features . But for the basic functionality I feel it should work . (accepting sms from client and sending out response dlr etc ..)
2) Has the library any restrictions? Support the whole features that
the smpp protocol defines?
Some of the commands are not been incorporated but you can implement it such as query_sm ,cancel_sm etc ..
3) Supports the creation of custom optional parameters ? How can I
define a custom optional parameter?
Yes you can use optional parameter ..