Markov chain: join states in Transition Matrix

Question

I need to merge two states in the Transition Matrix:

For example: I have the matrix below

          A       B       C      D      E      F 
      A  0.5     0.4      0      0      0.1    0

      B  0.5     0.1      0.2    0.1    0.1    0

      C  0       0.1      0.9    0      0      0

      D  0       0        0      0.7    0.3    0

      E  0       0.2      0      0.7    0      0.1

      F  0       0        0      0.5    0      0.5

And I want to join the states $D$ and $E$:

          A      B        C    (D+E)    F 
      A  0.5     0.4      0      ?      0

      B  0.5     0.1      0.2    ?      0

      C  0       0.1      0.9    ?      0

   (D+E) ?       ?        ?      ?      ?

      F  0       0        0      ?      0.5

What are the formulas to obtain the row and column $(D+E)$?

Using the constraint: "the sum over column must be equal to $1$" is simple to calculate the elements: $$ (A,(D+E))=0.2 \\ (B,(D+E))=0.2\\ (C,(D+E))=0.1\\ (F,(D+E))=0.5\\ $$ How can I calculate the elements of row $((D+E),i)$?