What I do is download the latest ESXi build and then insert all the VIB's I need. That is my preferred method and it is pretty straight forward. If HP already has an offline depot that is the route I would go. I was unable to figure out how to import the vendor ISO's into imagebuilder.
Currently, I do this with the dell OMSA plugin.
Here are my commands to add the dell VIB to the lateset ESXI and export it:
Image Builder
Add-EsxSoftwareDepot
https://hostupdate.vmware.com/software/VUM/PRODUCTION/main/vmw-depot-index.xml
New-EsxImageProfile -CloneProfile ESXi-5.1.0-20130504001-standard -Name "New"
Add-EsxSoftwareDepot -d .\OM-SrvAdmin-Dell-Web-7.3.0-333_A00.VIB-ESX51i.zip
Add-EsxSoftwarePackage -ImageProfile New -SoftwarePackage OpenManage
Export-ESXImageProfile -ImageProfile New -exporttoiso .\new.iso
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