I want to know how to enter Simplified Chinese Characters into a text document in English.
Possibly you need Insert|Special Character?
Just type them.
Of course, this depends on your OS (not mentioned) input method. If you want a more fine-grained control formatting, ou might consider to specifically assign some user-defined character style to the Chinese sequences.
But you are in multi-lingual context. This requires first to inform Writer through Tools>Options, Language Settings>Languages where you enable Asian language management. After enabling the feature, press OK. Reopen Tools>Options. You now have additional items Language Settings>Asian Layout and Basic Fonts (Asian) which allow more customisations.
Note that styles now allow you to set separately the font for “western” and Asian text.