They are auxiliary glyphs (輔助字形).
According to the wikibooks the rules to input them are as follows.
The basic glyph (radical) that forms a word just takes ONE code.
For example, 日 is input with the key "A".
An auxiliary glyph that forms a word should be decomposed according to the rules of decomposition.
For example, ⺜ is split into 冂(B), 一(M) and 一(M).
However, there are two more requirements in order to input them.
It is coded in Unicode.
(otherwise you have to make your own fonts)
The input method engine you use includes it.
(or you can make your own table)
⺜, ⺝ and 冃 are encoded and can be input with the keys BMM.
must be decomposed into 囗(W) and 丨(L).
Since it is not encoded, it cannot be input directly.