I believe you can create a calendar shortcut via a hyperlink. Similar to the mailto: hyperlink.
I am pretty sure I have seen this but you would have to research Outlook Shrotcuts. If you can create one from a hyperlink and pass some values then it can be done via a hyperlink control and concatenation in Infopath.
The is would pop up a new calendar item window and they would just need to save it.
To do more than that would require full-trust of your form which is not ideal since then you have to install the form.