Providing you have the access to Purchaser Categories, you are able to add multiple categories at once.
To add multiple, follow the steps below.
Click the Admin Cogs
icon along the top bar.
Click Products and Categories, then click Purchaser Categories.
Click down into the Category you wish to update.
Scroll down to the bottom of the page.
Under Add Multiple Categories header, separate each item on a new line.
Click on Add Multiple button.