Learn better Javascript. With Javascript and executeScript or executeScript_sandbox.
See executeScript, execute script, Arrays - Selenium IDE Commands Tutorial
Example:
{
"Name": "replace-variable",
"CreationDate": "2019-12-9",
"Commands": [
{
"Command": "store",
"Target": "var newval = \"\";var m = [\"\",\"January\",\"February\",\"March\",\"April\",\"May\",\"June\",\"July\",\"August\",\"September\",\"October\",\"November\",\"December\"];for(var i=0;i<m.length;i++){if(i == mycheck){newval = m[i];}}",
"Value": "replace_month"
},
{
"Command": "store",
"Target": "5",
"Value": "Var1"
},
{
"Command": "executeScript_Sandbox",
"Target": "var mycheck = Number ${Var1};eval ${replace_month};return newval;",
"Value": "shownotification"
},
{
"Command": "echo",
"Target": "${shownotification}",
"Value": ""
}
]
}