Here is what the conditions say about third-party goods in land mode:

Note that third-party goods (transported but not owned by NEXUS members) are not part of personal exemption limits. When travelling into Canada with third party goods, members must:

• Have the owner’s written permission to transport the goods on their behalf;
• Make an oral declaration;
• Provide documentation including receipts and invoices; and
• Pay all applicable duties and taxes for those goods.

You should be able to do this in a regular lane, but I would not try it in a Nexus lane.

The most definitive statement of conditions seems to be on the CBSA Nexus page. Canadian residents who are Nexus members can use a TDC card to declare goods for personal use when entering Canada in a Nexus lane. Others members entering Canada can only use the Nexus lane when they are within their personal exemption. Members entering the US in a Nexus lane can only bring goods for their personal use and must be within their personal exemption.

You can not use a Nexus lane with commercial goods or $10K or more in currency or other monetary instruments.

I think that it's strange that they go into detail about how third-party goods are handled before making it clear that you can only use the Nexus lane with personal goods. Viewers of Border Security may recall the story about the Nexus member who nearly lost his membership for bringing third-party goods into the US in a Nexus lane.