First, I believe in order to view the "Export to Excel" icon that the user must have Excel installed to their workstation ( as well as security rights to export from a lookup).

I do not think there is a way around that. If the user is a 100% Google Sheets office and does not want to use/license Microsoft Excel then I think that is a problem because they won't see an Excel export icon on their lookups due to Excel not being installed to the workstation.

I haven't tried it but think you might need this extension but probably still have to work 

You can open an Excel file in Google Sheets. The part that is going to be difficult is anywhere that Sage requires Excel to be loaded such as to export a lookup list.

Confirming what Wayne said regarding Excel being required to be installed for the Export To Excel button to appear in a lookup.

There are other ways to get data from Sage 100 such as VI Export, ODBC, Out-Of-Box reports on the menu with a "form code"/"report setting" or "Custom" reports added to the menu created to export to CSV, Tab Delimited, or Excel.

The above all work but it sure is convenient being able to tailor a lookup and just hit the Export To Excel button.

Sage is tied to Microsoft products. You can't use the built-in "Export to Excel" button/option in lookups or in Explorer if you don't have Excel. You'd have to export the data another way.