Short answer: no.
Long answer: It's not easy, but it is possible, if you're willing to work for it, and accept the limitations that would arise from such a hack.
There are two basic ways to accomplish this goal.
Solution # 1
The first way would be to write an extension for salesforce. This is browser specific, and has to be installed in each browser that wants the new features. This may be as simple as a GreaseMonkey script for browsers that support GreaseMonkey, or browser-specific extensions, such as Google and Firefox APIs. I wouldn't recommend this path if you're attached to Microsoft, Safari, Opera, or most other browsers that don't have this style of progarmming available. Conversely, if Google and Mozilla are acceptable norms, you can easily get something up and running, but you'll still pay in maintenance. Keep in mind you'll have to regression test every single release, because support for whatever you attempt might be dropped arbitrarily.
Solution # 2
The second way, which is what I've done in prior projects, is to write a Visualforce page. This has the benefit of being supported for years to come in a consistent, cross-browser fashion, requires only modest maintenance every few releases, and weighs in at just a few hundred to a few thousand lines of code that you'd have to personally write. Now, the bad news: you'll be limited to 1,000 emails per day, so you can't use this all the time in "busy" orgs. Also, templates become non-trivial to process, and would probably limit your email templates to plain text and custom HTML. Despite the harsh limitations, if you're willing to accept those limitations, this is actually the easier route to go down.
In the meantime, there's some buzz about a new Improved HTML email editor, so go vote for this feature so the product team can prioritize including a newer, modern editor.