There isn't a built in feature to merge multiple training plans into one plan but you can work around it by applying parts of two or more different training plans onto an athlete account and then exporting those workouts back to a training plan.
Watch the video or follow the steps to complete the merge.
Open the first training plan you want to merge in build mode
Click the "Apply to Clients" button
3. Choose the athlete you want to apply the plan to. It's best to use a test athlete rather than a real client. Then select a start date and the number of weeks of the plan you want to apply if you aren't merging the entire plan.
4. Click the "Apply to Schedule" button.
5. Now repeat the process with the second plan that you merge and apply it to the same athlete but starting on the Monday after the first plan will finish.
6. Now open the athlete calendar you just applied the two plans to. You will see you should now have parts of each plan applied to the calendar.
7. Click the "Export to Training Plan" button.
8. Now select the full date range you have applied. Give the plan a name and click "Export"
This will now create the new plan with the merged parts of the original plans. The new plan will now be in your plans list to give it a full description and set the other properties or customize it further.