This document details the procedure in using Microsoft Word as editor in SAP Script / Smart Forms.
In normal case, when you try opening the editor in Smart Forms, the following editor would appear:

Click on the editor button. The following editor appears:

Now to change the above editor to MS Word, do the following:
Go to transaction I18N (Internationalization).
Click on I18N Menu -> I18N Customizing ->I18N System Configuration (as shown in the screenshot

The following screen appears:

If you would like to use MS Word Editor in both Script and Smartforms, then check the both checkboxes
as shown below:

Click on Activate.
The following popup appears (related to SAPScript Editor):

Click on Yes. Now the following popup appears (related to Smartforms Editor):

Click on Yes to proceed.
Now the MS Word editor is available for both SAP Script and Smartforms.
Result: In the SmartForms:

In SAP Script:

You can anytime revert to old editor by deactivation MS Word editor option using the transaction I18N.