I'm not sure what you're trying to accomplish here (you haven't explained why you need to store a weaker type than you expose through your API), but to answer your question as asked, you can do:
Expression<Func<TModelMap, TNodeModelMap>> source = ...
var convertedBody = Expression.Convert(source.Body, typeof(object));
var convertedExpression = Expression.Lambda<Func<TModelMap, object>>
(convertedBody, source.Parameters);
RemovedProperties.Add(convertedExpression);