You need the following:
1) Enable WordPress Multisite in your main domain
- The Ultimate WordPress Multi Site Network Management Guide (tutorial)
- Multisite Rationale (case study, tips, tools)
2) Install the plugin WordPress MU Domain Mapping
- Multisite Domain Mapping Tutorial
- Here’s a workaround to make Domain Mapping work in GoDaddy and Dreamhost shared hosting (may apply to other hosting companies)
After that, you’ll be able to achieve your goal.