If your document fulfills the following requirements this should be easy to do just by using the standard extension mechanism available via the Nuxeo Platform:

Your custom document has a primary binary document of MIME type "application/pdf"
This is because this signature plugin signs only PDF documents


Your custom document is associated with the "files" schema
You need this for storing old versions of signed binaries

Here is how you would activate the “Signature” tab on your custom document type: <extension target=“org.nuxeo.ecm.platform.actions.ActionService”

    point="actions">

    <action id="sign_view_custom" link="/incl/tabs/sign_view.xhtml"
        enabled="true" label="Signature" icon="/icons/certificates.png" order="60">
        <category>VIEW_ACTION_LIST</category>
        <filter id="signature_filter">
            <rule grant="true">
                <type>File</type>
                <type>MY_CUSTOM_DOCUMENT_ID</type>

Thanks Wojciech, but I'm still having trouble connecting my electronic signature to a custom document… I've tried using your code but I keep getting an error message.. Help!