You cannot add or remove variations after an experiment has been started but you can clone any experiment to start with an exact draft copy of the experiment and add or remove variations from the newly created experiment as needed. Follow the steps below to clone an experiment:
To do this, choose your desired Project and select "Experiences":
Hover the mouse over the experience row you are interested in and select the dots sign and from there the option Clone Experience:
This will automatically create a clone of your selected experiment. You can make changes and then activate when you're ready.
You can also copy variation code (content) from one experiment to the other by selecting the variation code in the editor (see screenshot below)