According to the documentation, it must be possible to write to a linked table from the nms: recipient:
Here in the example I try to create a recipient and at the same time write in the table "Communication data" such as email and telephone number.
The cus:Kommunikationsdaten data is linked to the nms: recipient via the customer number:
<element integrity="neutral" label="Recipient" name="Recipient" revIntegrity="neutral"revLabel="Kommunikationsdaten" target="nms:recipient" type="link"><join xpath-dst="@Kundennummer" xpath-src="@Kundennummer"/></element>
Why does it not work with this soap call:
Check your SOAP request: the Kundennummer value is different in the recipient and KommunikationsDaten elements.
Once same, it should work.
Let me know if that helped.