Not currently, but you could build a web service that exposed that functionality, and deploy it as a plugin.

Looking in the admin console screen for managing user relationships (People -- Management -- User Relationships), it's the manage-relationships action that handles user relationships:

http://localhost:8080/clearspace_community/admin/manage-relationships.jspa

Looking in struts-community-admin.xml, it's the UserRelationshipManagementAction class that handles the backend processing, and manage-relationships.ftl that generates the admin console screen's html.

       <action name="manage-relationships"
                class="com.jivesoftware.community.action.admin.UserRelationshipManagementAction">
            <result name="success" type="freemarker">
                /template/global/admin/manage-relationships.ftl
            </result>
        </action>

Also, looking at the form in manage-relationships.ftl, the javascript method that calls the manage-relationships action specifies the addRelationship method, which would be a good place to look for exposing a web service

        function addRelationship(field){
            field.form.method = "POST";
            field.form.action = "<@s.url action='manage-relationships' method='addRelationship'/>";
            field.form.submit();
        }