C# Dictionary Add/Set in one call for performance reasons

In a Dictionary<struct,int>: is it possible to Add/Set in one call?

So is it possible to do the code below in only one lookup per entry?

_KeyToPoints = new Dictionary<Key,int>();

foreach ( var entry in billionEntries )
{
    int originalValue;

    // first lookup
    _KeyToPoints.TryGetValue(entry.Key, out originalValue);

    // second lookup
    _KeyToPoints[key] = originalValue + points;    
}

This is done in a very tight loop over a huge amount of data, so all performance matters.

Or is there a better suitable data structure?