Hello Carolina,
The "seclogin" switch is used to add a credential in the list, i.e. specify an user ID that can open the PSE file.
If you have a PSE file and want to import the certificate response (taking for granted that you have a SSL signed certificate), then you can use:
sapgenpse import_own_cert -c <certfile> -p <psename> -r <intermediate> -r <root> -v
<certfile> is the file that holds the certificate response
<psename> is the PSE filename
<intermediate> and <root> are files that holds the certificates from the CA that signed the request.
You can always use sapgenpse -h for more details on the switches available, and sapgenpse <switch> -h for help message in a specific switch.
I hope this helps,
Cris