This is not directly true, at least based on the version your using.
It’s a little bit complicated, but possible. At least at <=1.5.0.
The url you need to “call” looks similiar to this:
/check_mk/wato.py?sign=all&mode=agents&_do_sign=yes&key_p_key=%s&key_p_passphrase=%s&_transid=-1&_username=%s&_secret=%s" % (key_id, key_passphrase, wato_user, wato_pw)
The key_id needed to be look up once for each key at your site.
First pic the key you want to use for signing:
Then you need to examine the properties of the html-site and need to look for “key_p_key”.
If you have all information, then you can call the url above with the values and auto-sign your agents.
As soon as you change your signing-key, you need to look again for the changed key_id.
Greets