This wiki details about modifying important system properties online using Netweaver Administrator instead of using Configtool.
Technical Details :
Login to NWA using http://<hostname:port>/nwa with sufficient privileges as admin.
1. Choose Configuration -> Infrastructure -> Java System Properties.
Open the entry with the active template,then select the ID . Select the entry ThreadManager in the tab page Kernel. Search for the entries for which you need to see values.Example InitialThreadCount in the list of Properties.
Verify the Modifiable Online column check box of the specific property which details whether it can be modified online from NWA or only it should be changed from Config tool.If it can be modified online,choose Modify and set the custom value and save.
2.. Check the value of the maxHeapSize for all instances.
Choose tab page MemoryVM Parameters and find the vale for the entry maxHeapSize. Select the entry maxHeapSize and then choose Modify to set to custom value.
In this way , most of the system properties set from configtool can be seen from here and few can be changed as well.