Since version 2000, Outlook supports VBA. Unfortunately VBA support in Outlook is a bit poorer than in Excel or Word: You can only have one VBA project. Therefore you have to insert additional modules into the project manually and maybe add some references to the project manually.
To accomplish this, go to Outlook 2000 > Tools > Macros > Visual Basic Editor. There choose File > Import File for all files to be added. To add references go to Tools > References in the VBA window and choose the DLLs, OCXs, or TypeLibs.
If you want easier access to a macro defined in a module, you can e.g. add a toolbar button and assign the macro to it. Toolbar buttons can be added to Explorer's toolbar or to the Inspector's toolbar. The Explorer is Outlook's main window showing the item list. An Inspector is the window for displaying the processing an item, e.g. a mail. A pitfall with customizing an inspector's toolbar is that every inspector is affected regardless of the item type. So, if you add a toolbar button to the inspector for mail messages, this button also appears next time when you open an appointment.
To assign a macro to a toolbar button proceed as follows: Choose View > Toolbars > Customize either on the explorer's or the inspector's toolbar. Choose the Commands tab. Choose the category Macros. In the right pane, all available macros appears. Take the desired one an drag it onto the toolbar wherever you want. While the Customize dialog is still open, you can modify the toolbar buttons text label and even the button image via right-clicking the mouse.