There are a few different ways to add a new ad set to an existing campaign in StitcherAds.
You can choose to duplicate an existing ad set by selecting the ad set and clicking 'Duplicate'. You can then insert this ad set into the same campaign, an existing campaign or create a new campaign. This will copy over your ad set as it currently exists with targeting, bid & budget, and ads within the ad set. Once the ad set is copied over, any edits can then be made to the ad set or ad level.
Another way to insert a new ad set into an existing campaign is to build a new ad set through the campaign workflow by choosing a campaign type under 'Create Campaigns'. You'll see an option to 'Add ad sets and budget to an existing campaign' under the Campaign & Budget section of the workflow. From here, the list of campaigns available will be filtered on those that share the same objective as your new ad set.
A third way to do this is by using the copy campaign feature. This can be done by selecting a campaign on the campaign level and choosing 'Copy & Edit' or from within a campaign with the same 'Copy & Edit' function found to the left-hand side of the graph. This will load up the original ad set created for this campaign in the campaign creation work flow and any edits to targeting and creative can be made here before adding your new ad set into an existing campaign.
From the campaign level:
Within an existing campaign: