filling holes in steel

Yes, they can be filled. I wouldn't recommend it, tho. Most likely easier, faster, safer, and cheaper to replace the plates.
I will presume you are talking about a mild, commodity steeel such as A36. If it is a specialty steel, my concerns would be increased.
I have several reasons: (opinions may, and likely will, vary)
The plates take quite a beating, and will be worked where they are imacted. The embrittles the steel. Whee weld metal is applied, this will be annealed, but elsewhere it won't. As the steel is becomes more brittle, is is more likely to fail. This may not be by cracking visibly or outright breaking, but by spalling shards off. Not likely to hurt anyone, but could lead to failure at an inopportune time.
Where they plates have been hit, there is likely a lot of contamination (lead, which causes a lot of trouble when welded), copper, and other jacketing materials as well. They will be embedded in they surface of th metal whee there are flaws, as well as by the impact energy (lots of force, and lots of heat. The contamanants will alloy at the surface) Thourough prep by grinding will remove take care of this, but how much is your time worth? A sheet (4X10) of 1/4" A36 costs about 2 or 3 hours for me at my best local supplier.
Even experinced welders have trouble controlling distortion when doing this type of repair. If the plates become distorted or puckered, will they be acceptable? This really is a serious concern, as a small area can apply tremendous force to bend the plate as the weld cools.
Now, if oyu deide to repair the sheets, one method (easiest) would be to pad the areas and grind smooth. Generally, a pad is done by running stringer beads across the area, each bead overlapping the previous one by half of its width. If one layer isn't enough, another can be layed on top. Preheat is useful for controlling distortion.
If the pits ae small enough, they can be rosetted. This requires more skill, but uses less weld metal and is faster.
Padding is fastest with MIG or fluxcore, but requires a fairly high skill level to get solid metal (Iv'e watched a lot of 'good' welders lay crummy pads with MIG, even when they can run great groove welds.) I would not use MIG or fluxcore for rosetting -- too easy to leave pores or trap slag with fluxcore, leaving a place for failure to begin.
I don't know if I would want a filler with high impact strength or not- Might help with longevity.
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Thanks for the input.
I ended up talking to a local welder, and he was able to fill in the dings on the riser post, and we made a new plate from A36. That was his suggestion, I gave him no input such as yours. Took it to the range and gave it a pounding, performed just like the original plate.