Hello Georgie,
It is possible to add additional domains to your hosting package by ordering them as IRs that overlay the main domain. There are several IR options avaliable, see here:
http://www.futurequest.net/Services/IR/
An IRO functions by displaying the exact same content as the main package, and each main package can have up to 10 IROs:
http://www.futurequest.net/Services/IR/#IRO
An IRM displays only the content that is in a particular directory of the main package it is under and appears as its own separate website, and each full hosting package can have up to 5 IRMs:
http://www.FutureQuest.net/Services/IR/#IRM
Additionally, you can have subdomains that act as IROs and IRMs, and those are described in detail here:
http://www.futurequest.net/Services/IR/#DS-IRO
http://www.futurequest.net/Services/IR/#DS-IRM
You will want to review the limitations of IRs here:
http://www.FutureQuest.net/Services/IR/#Limits
And the following Knowledgebase article explains how email works with a main domain and the IRs under it:
https://service.FutureQuest.net/kba58
Each IR carries a one-time $20 setup fee, with no recurring charges, and as long as the main account remains current and active on the FutureQuest servers your IR domains may remain with no extra fees whatsoever.
And if IRs are not a viable option for you due to the above limitations, you can always order your extra domains as full hosting packages under your existing account.
All orders can be placed from QuestAdmin (once you have created your login):
https://Secure.QuestAdmin.net/