how to buuld a list of target accounts

I would like to create a target list of accounts that meet certain criteria identified for a sales initiative.  Once I know which accounts meet the criteria, the plan is to build a report to see my firm's history in terms of sales opportunities, activity, relationships, etc. to further quality the accounts. Here’s where it gets tricky:

• The criteria we’re using to get the initial set of accounts is sourced outside of Salesforce, meaning it’s not a field or option to filter and report on.
• The particular criteria / data point is only relevant to a couple of users so I can’t really justify adding a field for it on the account page layout. Additionally, that creates a slippery slope as I don’t want to add a new field to accommodate every single sales initiative. This would result in extremely cluttered page layouts and we’d have difficulty maintaining all of that data.
• The campaigns object only allows me to enter contacts, which isn’t helpful.
• Even if I were to try to use leads, they would not contain all of the historical account data I need to report on. 

Does anyone have any suggestions on how I might approach this? I’m even willing to consider an app.