I found this resource which lists the Supported Excel types and UDF arguments:
.gif)
1. Types are in the System namespace. These are the only supported argument types.
2. The term "All numeric" refers to the following types: Double, Single, Int32, UInt32, Int16, UInt16, Byte, Sbyte. Specifically, Int64 and UInt64 are not supported types.
3. Date/time in Excel is a Double internally, but the representation (algorithm to encode date/time as Double) is different from the .NET Framework representation, and conversion is necessary. Therefore, UDF arguments support DateTime as an explicit type, and Excel Services convert any Excel Double into a .NET DateTime when the argument is of type DateTime (assuming that the Excel Double is actually a date/time value).
4. Scalar object arguments are not supported; only one-dimensional or two-dimensional object arrays are supported (see the "Arrays and Ranges" later in this article). This table row defines only how each cell in the range that is passed into the array is handled.