Hi Vura_1234,
Any string of XML can be submitted to DBXL Web Service. The Web Service is not InfoPath specific. We definitely use DBXL to submit our forms' metadata (examples: worflow, history, tags, resources, etc.). You will need to specify the DocType when you submit the XML (leaving it blank defaults to current doctype). In addition, you will need to either concatenate the XML string together using a rule or set it using code. If you want to roundtrip the secondary XML (update it), make sure you submit the entire XML - including PIs - since that is where the DocID and other information is stored.
Send us e-mail if you'd like a more specific example.