Can I use a fry sieve to steam vegetables in a pot? Does it work for the purpose of steaming collard greens? The pot lid doesn't seal when the sieve is in the pot.
Anything that keeps the product off the bottom of the pan, and allows steam to circulate, can be used to steam vegetables. In this case a picture would be helpful, because fry sieves come is all different shapes and sizes. As you point out, the key will be keeping the steam in the pot and around the vegetables. You might try rolling a kitchen towel, placing it around the rim of your pot, and then placing the lid on top of that. Be careful that the flame, or burner, and the towel do not come into contact. Do this after water is boiling and when you can turn the burner down. Keep an eye on it. This can form a seal in a steaming situation, where the lid alone won't do the job.
I am 99% certain that you can. There is nothing special about a steamer, beyond that it has holes such that the steam comes through them, and that it can be suspended, so the vegetables don't touch the water.
Colanders, "standard" sieves, purpose-built steamers all work; I don't see a reason why your "fry sieve" (which I imagine to be the basked using in fryers) won't work.