For example, I want to enter Pinyin "shui4" 说(try to persuade as in 游说), instead of "shuo1" 说(speak as in 说话).

You can modify (add/delete) the Pinyin of any character:

1. Choose "Dictionary/Phrase Manager" from the input menu of Chinese WP, or input-method button on Chinese input bar;
2. Click the "Pinyin..." button on the dialog box;
3. Enter the Chinese character 说 in "Hanzi" field, then Pinyin with tone such as "shui4" (try to persuade) in the "Pinyin" field;
4. Click the "Add" button.