<p>Is there a way to send an email from an iSeries terminal, adding an attachment from the IFS? I know how to send a normal email using the SNDDST utility.</p>

There is two main methods of doing this.
The Do-It-Yourselfer will use QtmmSendMail API
Otherwise there are several other free and purchased products that you could use. The ones I would recommend are:
MAILTOOL
- Currently $399. Depending on what you are trying to do, I would recommend
SplTool Suite
if you want to email out spool files.
RPGMail
- Free API making use of JavaMail