Hello Qamar,
You will need to delete the existing client from the SEPM, and then login to the client computer with the SamAccount name, and then the SEPM should sync up user and computer.
I was able to do this by doing the following on a windows 2012 AD server.
1) Used ADSI Edit to change the sAMAccountName attribute to 12345
2) Update the schema cache
3) on the SEPM, changed the SEP client to user mode and then deleted it
4) Logged onto a client machine in format domain\username i.e. bob\12345
5) Once the client connected to the SEPM, my user 12345 shows a green dot in the OU that i imported. NOTE: for my testing I imported the default AD Users container
Thanks,
JonDkauf