Yes, this is possible. The TA 900s would need to have a SIP trunk back to the 7100 and then you could configure the analog voice users to register with the sip-identity command. Here is a sample configuration of how to accomplish that:
voice user 4200
connect fxs 0/1
sip-identity 4200 T01 register auth-name 4200 password 1234
NOTE: In this example T01 would be the SIP trunk used to reach the 7100 from the TA 900. The 7100 would need to be configured with a SIP voice user of 4200 and a sip-authentication password of 1234 .
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