No, I'm rather sure you can define default.htm as a "starting" or indexing page as well.
You can try it by just opening the httpd configuration file and then just edit the line that define the index.html as start index. Just add default.htm in this line and it will work.
Just because the sme server is a sme server this will just work for a time. The automatic configuration routines will write over your manual changes after a while.
Then you have to find the right template frafment and edit this "the sme way" to make it a permanent change.
This is not really difficult, it just takes a little bit time to find the documentation about template changes and then to do the changes.
To set up a sme server is very quick and easy, but changes like this is a little bit more time consuming than most other Linux distroes.
First you have to identify the required changes in the configuration file, and then eventually try out that.
Then you have to identify the right teplate fragment, copy it over to the folder for modified template fragments, and then do the changes there.