If I understand you, you are trying to have an icon other than what is supplied with the files appear in conjunction with a Tim Hortons alert. I don't think that would be an issue if you ensure the icon (which should be in .bmp format) is named EXACTLY the same as the .cvs or .gpx file (depending on which format you use) with the exception of the file extension (.cvs or .gpx) and you have them together in the same folder. If you are one character, space, or upper/lower case out, it will not recognize it. You could also include an audible alert in .mp3 format in the same folder if you chose, but those tend to get annoying fast especially in the case of such things as stores, etc.
I have retailers like Tim Hortons loaded in my unit and have only installed one icon that works for hundreds of the same retailer.
If you are looking to link a specific icon to many POI's within Map Source you can do it this way.
Load the gpx file that contains the POI's that you want to associate with a symbol, click on the first in the list then go to the last in the list and whilst holding down the shift key click on the last one. This should then highlight all of the POI's then using your mouse right click and choose properties. You can then click on the dropdown list for the symbol and choose which symbol you want for all of the POI's. If you have a specific icon make sure that you have it in your Custom Waypoint Symbol directory and is named between 000.bmp and 063.bmp. Also make sure that you assign them a category. You can then rehighlight all of these POI's and right click again and copy, then open up your main gdb database and paste these to this so that they will be in your main working map source file.
Hope this helps