Game Character Hub enables you to merge RPG Maker XP tilesets together. This tool will append tilesets one after the other to create a larger tileset that contains all of them.
To use launch this command, click on File > Tileset merging.
A window shall appear. On the left side of the window, a list of tilesets to merge is displayed. On the right side of the window, you may see a preview of the selected tileset.
To add a tileset, simply click on Add tileset, and choose the appropriate file. To remove them, select the tileset of the list you want to remove, and click on Remove.
The order in which the tilesets appear in the left list is the order in which they will be appended one after the other to create the new tileset. This means that the topmost tileset of the list will be at the top of the new tileset, (and the second tileset of the list will be located just after the first one, and so on and so forth). You may rearrange the order of this list by selecting a tileset, and clicking on Move up or Move down.
Click on OK to create the new tileset. A window will appear to ask you where to save it.