[How to] enable system users on PROFTPD

[prepared]

Hello
My problem would be that you want to use the proftpd mysql users will be able to not only work but also gives users the system (linux user).
for example.: adduser test
home directory: / home / test
Name matching will not be because the mysql username @ domain based on the user.

But for some reason the simple system user can not connect to the proftpd.
Sql module is faulty?

Thank you in advance for your help.
prepared

PS: Sorry for the grammatical and linguistic errors.