What is the best way to convert Action<T> to Func<T,Tres>?

Wrap it in a delegate of type Func<T, TResult> with a dummy return value, e.g.

public static void Execute(Action<T> remoteCall)
{
    Execute(t => { remoteCall(t); return true; });
}

you are asking literally to pass something that doesn't supply a result to a function that requires it.
This is nonsensical.

You can easily convert any function of Form Action<T> to Func<T,TResult> if you are willing to supply some result value (either implicitly or explicitly)

Func<T,TResult> MakeDefault<T,TResult>(Action<T> action)
{
    return t =>  { action(t); return default(TResult);}; 
}

or

Func<T,TResult> MakeFixed<T,TResult>(Action<T> action, TResult result)
{
    return t =>  { action(t); return result; };
}