[Serusers] ser and directing traffic to diff sip blocks in asterisk

[Iqbal]

Hi

Is there a way of getting ser to send messages to asterisk but to match 
different blocks in sip.conf in asterisk
In sip.conf asterisk seems to match on
a) the username (From:)
b) the ip address
c) default

What i want to do is to based upon certain matches in ser to direct the 
INVITE to a particular block in sip.conf, from where I can then match to 
a diff contect in extensions.conf. If this is not possible can I have a 
variable name for context based upon username of something like that.

I know this post would be better in asterisk mailing list, I will get 
there soon :-)

iqbal