This is not a simple task but it is possible. I'm not sure if you want the solution to be dynamic (updated automatically by extracting the current list from Excel) or static but you would need to develop this in stages and learn how to do this with a static solution first. Once you have the knowledge of how to do this with a static shape, you can build on that to get more automation.

To help you learn how tasks like this can be done it is important to start off with correct terminology so your web searches can be more productive. When you say 'Symbols that I can store in a template' I think the terminology is not quite right.

A template is a Visio document which is used to create new Visio documents
A Visio document can contain shapes but usually a shape is stored in a special library-type file called a Stencil. Stencils can be accessed by any document which makes the shapes in that stencil available to the document.
A shape is what you appear to be calling a 'symbol'.

Now, when you talk about accessing Excel for a dropdown menu, this is possible but would be a strange thing to do since it would take 5-10 seconds to open and interrogate the Excel workbook and this is a lot slower than most users would be prepared to wait. But you could import the Excel data (if you have Visio Professional) to make that part a lot more responsive.

I think you might need to post more details (like Excel and Visio docs) so I can see what it is that you want. I'm not sure how the 'symbol' is going to come from Excel.

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Andrew Lockton
Chrysalis Design, Melbourne Australia