I can show you how to do it in DL, but I think the file structure has changed somewhat with the expansions. You'll just have to go in and see if it works the same way.
I use Hexplorer. First, go into View/Options and set the font to Fixed Roman Large. Set 'Column Number' to 20 and 'Group columns by' to 4. Doing so will make it easier to read, as well as aligning the columns so that they are not staggered.
It is best to edit the planet in question just after colonizing, but before adding anything. It should contain all blank tiles.
Open the file, then click Edit/Find. Enter the planet name (don't forget to capitalize), but not the planet number. Just hit F3 three times to get to the next planet number. So if you are going to edit Frier 3, search for Frier and then hit F3 6 times and you will be at the beginning of the Frier 3 planet code.
Tiles begin with 803F.
Scroll down until you see this sequence:
Copy the code for the bonus tile that you wish to add (I'll give you the list of bonus tiles at the end. You can copy and paste it to a Notepad file for safe keeping. Then simply double-click to select the line of code and copy it.), then select a tile code block in Hexplorer by click/drag:
Right-click the selection and choose 'Paste external text / as hex numbers' from the context menu:
The end result will look like this:
That particular entry will add a 700% Research tile.
If your planet already has bonus tiles, be careful that you don't overwrite it.
When you see a block that begins 803FFFFFFFFF - that is a tile that is not yet available, but can/will be (once you terraform the planet). It is safe (even preferable) to use that block to add your bonus tile. Doing so will simply add more usable tiles to the planet.
Here is the list of bonus tiles: