I'm using imscp's provided plugins for spamassassin (sa) as well as opendkim. In config.php you stated, that dkim should not be activated if opendkim-plugin is already active. Why? Currently, opendkim signs messages or checks, if a signature is valid - but usually nothing happens if a signature is invalid. What about letting sa to perform dkim-checks as well in order to (potentially) increase spam-value?
Or did I get something wrong? 
Thanks in advance.
By default, the opendkim plugin acts as signer and verifier 
So here, DKIM plugin from SpamAssassin would be useless. However, if you configure opendkim plugin as signer only, you can enable the DKIM SpamAssassin plugin. See the OpenDKIM plugin configuration file.
Currently, opendkim signs messages or checks, if a signature is valid - but usually nothing happens if a signature is invalid.
That not true OpenDKIM can reject message
What about letting sa to perform dkim-checks as well in order to (potentially) increase spam-value?
You can of course, in this case, make OpenDKIM acting as signer only (see the OpenDKIM configuration file). Don't forget to update the plugin list after editing the config.php file, else your change won't be taken into account.
Hmm, I didn't encounter any event so far, where opendkim rejected an email I think I'm gonna give SA's dkim-checks a try.