Hi,
Unfortunately this is not a feature we (or anyone I am aware of) provide.
Our systems can support the use of FC LUNs from other SANs as a backend for volumes we present, either under V-Series or FlexArray, but both are licensable features and only supported on some of our platforms.
The easiest option may be to contact your local NetApp account representative about options of competitive takeout of your other SAN and install more storage for your NetApp system.