Most colleges do not want external LORs or extra ones. They want to keep it a level playing field. The GC LOR is considered the most important as that person assesses a lot of students from your school over the years.
Colleges do not look at non confidential LORS the same way as they do the blind ones for the obvious reasonS that it’s more difficult for a recommender to be candid when their comments are being monitored by the person getting the recommendation, and it also allows students to get a bunch of recs and pick.
If you have some sterling LORs from the outside, you can give copies to your GC and let them look at them and mention high points from them. In fact, you can write out those high points on the sheet they provide you and just include the letters as reference if they choose to check it out.
If you were that amazing at a college program, and you want to attend that college, you can ask the professor of that course if s/he’d put in a word for you at admissions. Though if you were that amazing, they may do so unbidden. I’ve known a few cases where thst has happened but it is very rare.