Generally, the short answer is no. There is simply no facility to output as .HTML and not .HTM.
Would I be correct in assuming RoboHelp is new to you? I say this because you mention all the calls are looking for .HTML and not .HTM. This seems to suggest something other than RoboHelp was used before.
I suppose you could attempt renaming all your topics. But if you choose to try that, you seriously need to do it only from inside RoboHelp. Otherwise you will be asking for some trouble.
Yours wouldn't be the first time I've seen folks as for this possibility. You (and others) should consider submitting a Wish Form to ask the development team to consider adding this as a feature. (Link is in my sig)
Helpful and Handy Links
This is clearly related to your other post and it would have useful to mention that there. If your developers will not even change a call from HTML to HTM, you are going to have an even bigger problem getting them to call help from a CHM.
See www.grainge.org for RoboHelp and Authoring tips
they are willing to change the calls but I just wanted to make sure they had to.
as to my other post peter, just another reason why they will need toc hange the calls anyway.