If you are already using Wordpress you could just put the header.php and footer.php into the header.tpl and footer.tpl of WHMCS, make sure you dont remove the whmcs navigation from the header and then add and link the CSS file from your Wordpress, do some CSS tweaks to existing whmcs to fix any clashes and it should be fine.
If you need some more help you can PM me as I know it probably sounds pretty confusing but it's actually quite simple once you get the hang of it.
First you need to decide if you will do it yourself or not. If you will WHMCS has provided documentation which should help.
If not then you could either hire someone to complete the integration for you (search Google for WHMCS integration) or you could use a plugin as suggested. Both options have their advantages and disadvantages but should acheieve similar results.
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