Yes, It is possible to set deferent optimization option each file.
[When using LEXIDE-U16] Select the source file on the project tree on the [Project Explorer], select [Properties] on the pop-up menu displayed by right-clicking. Specify the optimization option on [C/C++ Build]>[Settings]>[Tool Settings] of [Properties] dialog.
[When using IDEU8] Define multiple option sets in advance, select source file on the project tree, select the option set on the pop-up menu displayed by right-clicking. For details of the definition of the option set, see "4.6 Defining Option Sets" of "IDEU8 User's Manual".
