User Defined Aggregate Functions without DLL
No. User-Defined Aggregates (UDAs) can only exist as SQLCLR objects. There is no T-SQL equivalent. The same holds true for User-Defined Types (UDTs).

That answer assumes the question is actually asking if UDAs require an Assembly as created via CREATE ASSEMBLY.

However, if one interprets the question as asking if a DLL file is required by the CREATE ASSEMBLY statement, then no: you can create an Assembly from either a DLL on the file system or from a VARBINARY literal or variable containing the same bytes that would be in the DLL:

CREATE ASSEMBLY [AssemblyName] FROM 0x1234A6B3......;