Yes, you can do this. Open the Mail app, then press Menu, More, New Account, Other (Pop3/IMAP); press Manual setup, leave the Protocol on POP; enter the email address, e.g. myname.gmail.com; enter your user name and password, enter the POP server as pop.gmail.com, the security type as SSL, port as 995; and press Next. After it has verified your account information you enter smtp.gmail.com for the SMTP server, TLS for the Security type, and 587 for the Port; press Next. Select "Make this my default mail account" if that's what you want, then press "Finish setup".
Now, the big problem with this is that if the Gmail app gets to your mail before the HTC Mail one does, you won't get to see it in the HTC one. So you need to go to your Home screen, press Menu, then Settings, Accounts and sync, select Google, then deselect the Sync Gmail option. This will only stop the Gmail app syncing. The HTC one should sync with all of the accounts you set up in it.
Note that I haven't actually tried all of this myself as I have no intention of having my Gmail come down into my HTC Mail box; I want to keep them both separate. But I don't know of any reason why this won't work. Also, this solution assumes that you want to actually download your mail into the HTC Mail app using POP3, the standard protocol. You might prefer to use IMAP instead, in which case the HTC Mail app will simply synchronise with your Gmail account so that your inbox shows what's actually on the IMAP server, and so that you can interact with it directly from your phone rather than downloading the individual emails (this is a little like using Gmail from the web interface, but easier.
If you want to go this route, look at
this web page. When you've read down to the bit that says, "follow the configuration instructions", click it and you'll see that Android is supported and that there is a link under the heading "Mobile devices" that shows you how to set it up.
For those who want all of their mail in the one interface, this seems like the best option: POP for your non-Gmail accounts and IMAP for your Gmail account, all in the one place.