You have the possibility at step 7 of learning path builder to assign existing activities to the modules of your learning path.
For every module, you have 2 buttons available.
The first one makes possible to add one single activity:
After selecting an activity type, you can choose to either create a new one, or to reuse an existing one, by entering its title in the autocomplete field:
The second button, "Activities bank", makes possible to add several existing activities at once to your module:
You can check as many activities as you want, then click on the button "Add selected activities to the module" at the bottom of the interface: