Yes, it's one of the easiest merges.

In many cases, you can simply use nwhak to extract all the files from the haks, then load them into one big hak. This works because tileset files have unique names.

Occasionally, though, you will find a file with the same name in more than one hak.

The most common example is doortypes.2da, which is defines custom doors. There is a tutorial on the Vault on merging haks which explains how to merge 2da files with a text editor. Using the latest official EE doortypes.2da as a starting point, add any custom doors you find in hak versions of that file.

Another example is when you have two versions of the same tileset. If at all possible, try to choose the one that seems best, and forget the other. Merging them requires a fair amount of time and expertise, not to mention a lot of testing.

Other conflicts are rarer - best to ask if and when you encounter one.

Many thanks Proleric. Which one of these tutorials would you recommend? I know that you're an experienced builder, I value your opinion.

Well, you won't need a tutorial unless you hit the doortype.2da issue or similar. For that, if memory serves, this tutorial is OK:
https://neverwintervault.org/project/nwn1/other/hakpack-and-2da-merging-tutorial
with the caveat that for distribution to other players, it's good form to publish a merged hak rather than an override.

The Custom Content Guide (also from the Vault) is a good general reference for dipping into, skipping over the deeply technical bits, albeit light on tilesets.

NWN University (Vault) has most of the guides you'll ever need.

Well now, many thanks for these directions, I'll certainly look these ways.

I already managed to merge two tileset haks which had door files in them (using Notepad), and it works. When I loaded third tileset to the pack however it still loads, but I can't put any doors on the buildings. There was no .2da file in the files as well. What could be the issue here?

Which was the third tileset?

Ordinarily, you can paint standard doors in any tileset, even if doortypes.2da is a bit out of whack.