We're considering how to add a way to do this from the admin that's super smooth (you can see the feature request here and vote for it), but for now, here are your options:
Give your customer a one-time 100% off coupon and ask them to book it on the storefront. You can collect money from them any way you want after they book if necessary.
Use the storefront just like a customer would and take their payment information over the phone if it's a prepaid order. This is a very straightforward and easy way to book people. You can also use private coupons (e.g. "100% off") if they have already paid via other means.
You can create a draft order for the correct Experiences product, invoice the customer, and then use our Unassigned Orders function to assign them to the correct time slot. You'll be able to invoice them or apply a coupon/discount here as well.