Aircrack Dictionary Attack
Unfortunately, the answer is that currently, aircrack-ng does not support such a feature.
However, you could achieve the same result with john (John The Ripper) in combination with aircrack.
To start a session foo that you want to pause and resume later, execute:
$ sudo john --session=foo --stdout --wordlist=wordlist.dic | aircrack-ng -w - -b 00:11:22:33:44:55 WPAcrack.cap
To resume this session foo at a later time, run:
$ sudo john --restore=foo | aircrack-ng -w - -b 00:11:22:33:44:55 WPAcrack.cap
This will allow you to bypass the aircrack-ng limitation.