I do believe that the conversion can be done as prevoiusly described but I'd suggest just replacing the original equipment. The nitrogen cells are very easy to replace and not very expensive (probably around $150 for the pair on any of the decent online sites). I replaced mine at the beginning of the summer.
The shocks are also not too hard from what I've read here but that's not something that I've needed to do yet. Do a search and you should find several very good and detailed descriptions. If you come across one by thomspin (SP?) it will be excellent - he does a great job of documenting MB repairs.
So why stick with the original? The hydropneumatic system is designed to maintain the proper suspension height and attitude under almost all driving and load conditions. Given the performance that is possible with the 560, this can be important especially if you frequently travel on windy or less than ideal roads with several passengers or stuff in the trunk. I noticed a marked improvement on my car after replacing the cells - the car handled the same at speed through curves regardless of how many people were with me.
'87 560SEL 267K (177K on motor) Blue/Blue
'98 Buick LeSebre 60K (wife's car)
'56 Imperial Sedan 124K
'67 Dodge Monaco 130K (Sold)
'87 Chrysler 5th Ave 245K and going strong (sold)
'73 Plymouth Satillite 175K (sold)
'96 Chrysler LHS 80K (totaled)