when seed resources are transported from one planetoid to another inside of a command module, they are greyed out for planting on the target planetoid unless they are swept onto the target planetoid's storage first. They cannot be set to 'plant' if they are still inside of the rocket module.
Build rocket on planetoid B
Build Storage Bin inside rocket's command module
"Sweep" seeds into the storage bin
Travel to planetoid A
Attempt to plant seeds in a farmable tile/box, but get stopped because "plant" button is greyed out.
"Sweep" seeds from rocket command module to storage bin on the planetoid
Planting is now possible