You can simply do this by dragging one entry to another. Suppose your word list has
you can simply grab wanting or wanted with your mouse and place it on want.
(See choosing lemma file if you want to join these to a word which isn't in the list)
Can't see the word to join to?
If you cannot see all the items you want to join in one screen, you can do the same thing using by marking.
1. Use Alt+F5 to mark an entry for joining to another. The first one you mark will be the "head". For the moment, while you're still deciding which other entries belong with it, the edge of that row will be marked green. Any entries which you then decide to link with the head (by again pressing Alt+F5) will show they're marked too, in white. (If you change your mind you can press Shift+Alt+F5 and the marking will disappear.)
2. Use F4 to join all the entries which you've marked. The program will then put the joint frequencies of all the words you've marked with the frequency of the one you marked first (the head).
Alternatively, 1. select the head word, this makes it visible in the status bar.
2. Find the word you want to join and drag it to the status bar.
If you select an item which has lemmas visible at the right and press Ctrl+F4, this will unjoin the entries of that one lemma. To unjoin all lemmatised forms in the entire list, in the menu choose Edit | Join | Unjoin All.