Yes.  Select the items in Item Takeoff (Takeoff Tab>Item)  Click out of the item list which will activate the save as assembly button.  Give the assembly a number and description and then choose if you want to make the assembly a one time assembly (this estimate only) or a database assembly.  If you don't have formulas on your items you will need to add formulas to have the assembly calculate.

Thank you Gary.

Can it be done from items in the spreadsheet, I mean, selecting a few items already in the estimate and save them as an assembly?

Thank you Gary.

Can it be done from items in the spreadsheet, I mean, selecting a few items already in the estimate and save them as an assembly?

I don't believe this can be done. There are some great YouTube videos that explain a lot of this. just look up "sage estimating" or try this link Part 1 of 8 | Building Useful Assemblies In Sage Estimating SQL - YouTube

This isn't the exact video but it will get you pointed in the right direction.