# cli:
perl -pne 's/(.+?):.+/user_$. $1_$./' /etc/passwd

# or from file:

#!/usr/bin/perl -np
s/(.+?):.+/user_$. $1_$./
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Update: instead of "username_1 name_1" format you may want "username_1 name"
in which case substitute $1 for $1_$..