Outdoor potting

Any advise on how to add some more soil to already potted seedlings

Cut the bottom off the pot and stack it on top of another the same size, or on a hole in the ground full of the soil of your choice.

you can top dress that with new soil, right up to the bottom set of leaves, the stem will just start to grow roots eventually. just make sure to let it dry out good between watering, don't get the buried stem soggy till it changes....don't get it soggy then either, but you get it

if you let that dry out pretty good, you can just unpot it carefully, add some new soil to the bottom of the pot, then fill in around the sides after you replace the plant.

Wait for it to grow. Flip it up side down and transplant/add more soil to bottom.