if we have set up a medicaid trust and some of the contents include mutual funds, can we purchase additional shares of the mutual funds that are in the trust?

I assume you are asking about a special needs trust created to preserve a disabled person's eligibility for Medicaid.  Those trusts have very specific requirements that vary from state to state and are often under court supervision.  I suggest you ask the lawyer who set up the trust in the first place if you can make this transaction.