Using XML::XSH2, a wrapper around XML::LibXML:

open 1.xml ;
cd /dict ;
insert element key prepend . ;
set key[1] 'ADDKEY' ;
my $array := insert chunk '<array><dict/></array>' after key[1] ;
my $key   := insert element key into $array/dict ;
insert text 'SAMPLEKEY' into $key ;
my $array2 := insert element array after $key ;
set $array2/string 'STRING' ;

delete key[last()] ;    
xmove :r string[last()] before preceding-sibling::key[1] ;

save :b ;
[download]

I tried to demonstrate several ways of creating new nodes: via insert and set (more than one node can be created by both, see insert chunk and setting of string).