This should be doable but you may need a few extra steps. Take a looks at this thread about a similar situation: https://community.tableau.com/message/533716?et=watches.email.thread#533716
In general, the workflow in tabcmd should look like this (each step will be an instruction):
1. Login to Tableau Server.
tabcmd login -s http://yourTableauServer -t "SiteName" -u yourUsername -p yourPassword
2. Download the TDSX file (this is a data source file that includes the extract within it) that you want to append.
tabcmd get "/datasources/extractName.tdsx" --filename "C:/Temp/extractName.tdsx"
3. Unzip the TDSX file to get the TDE from it.
unzip -o C:\Temp\extractName.tdsx -d C:\Temp
4. Publish with the append option.
tabcmd publish "C:\Temp\Data\extractName.tde" -n "masterExtractName" -r "ProjectName" --append
Hope this helps!
Ah wicked, I'll try this out for sure!