Is it possible to construct a path without a seeker for a specific graph usign NNConstriant or am I required to create a seeker per graph (/agentRadius) for this?
The Seeker is just a helper class, you can skip it if you want.
See the ‘Using AstarPath directly’ section on this page: https://arongranberg.com/astar/docs/callingpathfinding.html
You can set the graph mask using for example
path.nnConstraint.graphMask = 1 << graphIndex;