That sound more like you would do using Footnotes (or endnotes, which are footnotes but collected at the end of the paper). Have you considered using a footnote style instead?
Could you further explain what you mean by a footnote style?
At the moment I just insert a citation via the endnote tool in word in the text as a number and then at the end of the document a bibliography of all the numbers with the corresponding papers is listed. Now I want to add text into the bibliography at the end of the document but keep the number in the text as is. When I just type text into the bibliography it is gone as soon as the bibliography is updated.
Yes, the Bibliography is an auto generated “field” and you can’t edit it unless you are finished writing, about to submitt to to the publisher, and want to unlink all the endnote stuff (convert to plain text - which makes it impossible to make further Endnote related changes, add new references,etc - so always do this to a copy, so you can return to the endnoted version for further editing).
If you are well into the writing it is a bit late to take the alternative direction (although I think there are Macros to do it) of using MSword’s insert footnote and put the citation into the footnote itself after you have inserted your text.
The other trick is to put your citation as usual, and then insert a separate note from insert citation ribbon option - but you would need to manually update teh number you referred to, so it would have to be there twice.
to be honest, the way I handle these instances - is to say in the intext citation with the prefix options, (For excellent reviews, see 1a-c )