I accidentally deleted part of my comment before submitting. Posting again here:
To the anonymous submitter: We don't automatically approve every commonly used expression. Two editors gave their views and left the submission open for others to comment. That's what the comments section is for. We can't read your mind. Your arguments for why a word/expression should be added are welcome. Snide and antagonistic remarks are not.

それはかわいそうに。I’m sorry to hear that. 
彼がかわいそうに。I feel sorry for him.

On reflection, I can see the case for including it but there are a couple of things to note.
かわいそうに is essentially an abbreviation of かわいそうに思う; it can take a subject or function as an interjection. In that respect, it is no different from かわいそう.
If "you cannot derive the meaning of this from the かわいそう entry", that suggests the かわいそう entry needs improving. It's the same word.
To the anonymous submitter: We cannot read your mind. Your arguments for why a word/expression should be added are welcome. Snide and antagonistic remarks are not.

@@ -19 +18,0 @@
-<gloss>poor you!</gloss>
@@ -20,0 +20,2 @@
+<gloss>pour soul!</gloss>
+<gloss>what a pity!</gloss>

可哀想に	134415
可哀相に	45447
可哀そうに	7376
かわいそうに	318672
GG5: まあかわいそうに! What a 「pity [shame]! | Poor 「man [girl, etc.]! | Poor 「thing [soul, creature, 
中辞典: かわいそうに! Poor thing [man, girl, etc.]!
Tanaka: かわいそうに、その少年は車にはねられた。 The poor boy was knocked down by a car.
Tanaka: かわいそうにその子供は餓死寸前だった。 The poor child was on the verge of starvation.

I'm comfortable with it as an entry. Obviously "uk". There are 23 Tanaka sentences with かわいそうに, and in most it seems to be used as a "Poor ..." adjective pattern, often as かわいそうにその....

@@ -17,0 +18 @@
+<misc>&uk;</misc>

I think this should definitely be an entry. It's surely a grammar point but a pretty damn 
obscure one. I can't think of any other similar expressions except (お)気の毒に. I also 
don't think this is worth this amount of arguing over it so I will just go ahead and 
approve it. If somebody disapproves, feel free to re-open the discussion of course.

I support adding the かわいそうに as an 
expression/interjection as its own entry. 
Its meaning is more than just かわいそう + 
に so the entry will be helpful for users. 
It's also very common. 

Plus, we do already have entries for the 
following:  

お大事に
焦らずに
お気の毒に

which are similar.