Is so called "Polymerized LiNb (Lithium Niobocene)" a possible compound?
Materials such as Terfenol-D and Galfenol are magnetostrictive ferrous alloys that expand and contract in the presence of magnetic fields, as well as creating magnetic fields when acted on by a physical force.  Both these materials were developed at Ames Lab (formerly the Naval Ordnance Laboratory, that's why the material names end with -NOL).  Terfenol-D has the highest magnetostriction of any Fe-alloy, but the high rare earth content or Terbium and Dysprosium limits its commercial applications.

The "polymer" moniker for Lithium polymer batteries does no refer to the lithium metal or oxides, it only speak to the polymer electrolyte or a polymer casing.  It just a marketing term, no a chemical designation

So on to your question... Polymer, no.  Monomer, no.  Niobium has an extremely high melting point (nearly 2500C/4500F).  It is a very difficult material to melt.  Lithium boils around 1300C.  From a metallurgical processing point of view, you would need a chamber pressurized to several MPa with an inert gas to prevent the lithium from volatilizing as you attempt to alloy it with niobium.  You would also need some powerful magnetic stirring to attempt to keep the mix homogeneous since the lithium would literally float out of the solution.