While pushing the data on People form use an AR System function LOWER in Push Fields action, which returns its argument in lower-case. You can also use it for multiple fields.
Example: If your First Name = "Darshan" and Last Name = "Gujrathi", then in Push Fields action you can use:
| 'Login Name' | LOWER($First Name$ + "." + $Last Name$) |
This will store the value of Login Name as "darshan.gujrathi"
Let me know if it's not clear to you.