Can we use DemandTools to compare data across external spreadsheets or with another CRM software package?
No. DemandTools for Salesforce only works with Salesforce Data. The Discovery -> Find/Report IDs module can be used to compare data from an external spreadsheet to data in a Salesforce table, but the tool cannot be used to compare data across external data sources and cannot be used with other CRM applications.
DemandTools for Dynamics is compatible with the Microsoft Dynamics CRM.