Copy the world you are using from tekkit/technic to vanilla.
Then install mystcraft, and you should have all your stuff from mystcraft, and non from the other mods.
How to find your tekkit/technic world (only works for windows)
1. Click the windows icon in the left bottom corner.
2. Type in: Run (only if you have english windows. Else type run in your own languge).
3. Then you should get a little box. Type in: %appdata%
4. Now click the folder: .Techniclaucher
5. Now you should see all the diffrent modpacks, technic includes. (if some of them is missing its because you havn't played them). Click technic or tekkit, depents on what modpack you play with.
6. Now click saves, and you should see all your worlds. Just drag the one you want, into vanilla minecraft. (In the save folder).
How to find vanilla minecraft saves (where you have to drag you tekkit/technic world to)
1. Click the windows icon in the left bottom corner.
2. Type in: Run (only if you have english windows. Else type run in your own languge).
3. Then you should get a little box. Type in: %appdata%
4. Click .Minecraft
5. Click saves, and drag you Tekkit/Technic world into there, and install Mystcraft and you are good to go :D