Hey Katy,
This is a bit of a long answer as I do not know exactly what use case you are looking to achieve but there are only two ways to 'lock" fields today:
1. Load in an entire line item as external and the entire line item is then not editable by anyone in the tool. Only admins can load this in.
2. Add an attribute to a reference dataset so every time x is chosen y automatically fills in and it not editable. For example when you choose account number in the tool the account name is auto populated and can not be changed. This only works if there is a one to one relationship with a key reference dataset attribute.
If the above is not what you are looking for then I am not sure i know of another way so you may want to search the ideas to see if it is already submitted and vote on it or you can create a new one.
Please let me know if you have any other questions,
Lauren Griessel