В BOL прямо написано для create default:
Syntax
CREATE DEFAULT default
AS constant_expression

Arguments
default

Is the name of the default. Default names must conform to the rules for identifiers. Specifying the default owner name is optional.

constant_expression

Is an expression that contains only constant values (it cannot include the names of any columns or other database objects). Any constant, built-in function, or mathematical expression can be used. Enclose character and date constants in single quotation marks ('); monetary, integer, and floating-point constants do not require quotation marks. Binary data must be preceded by 0x, and monetary data must be preceded by a dollar sign ($). The default value must be compatible with the column data type.

Т.е. нельзя...