Abundance is just a Measurement and you can set up your own to meet your requireemnt.
For example set up Meaurement through the Terms List for Type = Sex, Qualifier = None and Unit = None, Type = Stage, Qualifier = None and Unit = None, Type = Count, Qualifier = 'None and Unit = count (or whatever scale you use). Note after setting up the Type you will get a Measurement Qualifier and Measurement Unit shown below the type. The Qualifier and the type then need to b set up. Right click on each of these and click on Add/Child. The way thsi works isn't that obvious.
Use a separate column in the import for each type, Sex, Stage and count. For example you can put Male, Female, Unknown in the sex column, 'Adult or Juvenile in the stage column and the number seen in the Count column. Match the column to the type. They will will appear Taxon Occurrence Data for matching in the Import Wizard.
This will mean you have mutiple occurrences for the observation of just one species rather than one observation with mutiple abubdances. Not perthaps what everyone would want, but R6 can accommodate the approach.
Measurements are very versatile and can be used to add all sorts of additional information to a record.
Mike Weideli