I have many L1 resonators but can't deploy them anywhere because all portals are full. Can I upgrade them with XM or something similar? Even if I hack any portal I mostly only get L1 resonators although I am already at L3
If you don't need them, give it to your faction L1 players, so that they can deploy them and therefore level up.
L1 resonators are perfect to booby trap the portal, as it will get destroyed very fast especially if put close to the strongest resonator, the owner of the resonator will get notified once it is destroyed and can counter attack - recharge the portal remotely.