you would need a little machanical knowledge but the job itself is not hard.
You need a specal tool to remove the valve springs. The last time i did this job (on another car) i made i plug that fitted into the gluw plug (or injector) spot that i could hook up to my air compressor. You can them pump a little bit of air into the cylinder while you remove the spring. This insures the valve doesnt fall into the bore.
I am assuming you have 2.8 patrol and from what i understand there are a couple of things you need to know before removing your rocker cover. (vacuum pump is bolted on top)
I will almost certainly guarantee that it wont be just the valve stem seals that are causing the smoke. Its almost harder to replace them insitu than it is to take the head off and give everything a once over. Vlaves will be looking sad by the time valvestem seals wear, well the original nissan ones anyway.
but to answer your question yes it can be done like said by injecting compressed air into the cylinder as you remove the collet and spring.
An old method of removing the valves (once the head is off) if you do not have a valve tool is to place a large nut under the valve , the depth of which should allow the head to sit flush on the floor.
Grab a large open ended spanner and place it over one of the valves, it needs to be wide enough to allow access to the valve colletts, holding the spanner at its end in one hand stand on the middle of the spanner and push down on the valve, you may need a fine screw driver to flick out the colletts, but probably not, just keep foot pressure on valve till you get them out, I used this method for many years !....good luck